How do you add integers with different signs?

It's a simple process, all you do is just imagine it as money. Say you owe someone $10, which would be -10, an you give them $4, which is +4. You would still owe them $6, so your final answer would be -6.

the revenue of the fitness center, y, is the product of two factors: the number of members and the fee paid by each member. there are 320 members now, but number of members goes down by 10 for each $5 increase in fees, so if x is the number of $5 price increases then members is equal to (320 - 10x). since the fee is currently $45, and the fee increases by $5 for every price bump x, the fee is given by (45 + 5x). so, revenue y is given by y = (320 - 10x)(45 + 5x).